Syrian pro-government forces set up a road checkpoint next to a mural reading “Islamic State” in Arabic after they took control of the northern Syrian town of Maskanah on Monday. (George Ourfalian/AFP via Getty Images)

U.S.-backed forces have begun the “long and difficult” battle to capture the northern Syrian city of Raqqa, the Islamic State’s de facto capital, the U.S.-led coalition fighting the extremist group said Tuesday.

Kurdish-led militants began laying the groundwork for the ­offensive in November, edging through the surrounding province and cutting supply lines into the city. But a showdown for the city  itself will prove a major test for the coalition, with the potential for high civilian casualties.

“The fight for Raqqa will be long and difficult,” Lt. Gen. Steve Townsend, the coalition’s commanding general, said in a statement. In northeastern Syria, the U.S.-backed Syrian Democratic Forces (SDF), a group dominated by Syrian Kurdish militants, announced that a “great battle” had begun.

Islamic State militants seized Raqqa in January 2014, transforming it into the hub from which the group’s leadership planned expansion throughout the region and attacks around the world.

Three and a half years later, the city has diminished in importance as the group has lost two-thirds of its self-declared caliphate across Syria and Iraq. Western diplomats and experts monitoring the group say the Islamic State has relocated foot soldiers and senior leaders to the eastern province of Deir al-Zour, where an even tougher fight against the militants can be expected.

But U.S. officials estimate that at least 3,000 Islamic State fighters are still holed up inside Raqqa, where they have erected defenses against the anticipated assault.

Among them are as many as 200,000 civilians, who aid groups fear may be used as human shields, a tactic employed by the Islamic State in its strongholds across Syria and Iraq as coalition forces closed in.

Conditions inside the city are understood to be dire. According to a recent assessment by the Syria Relief Network, a coalition of nongovernmental organizations, two-thirds of the population is living on two meals a day amid dwindling supplies of essentials because of the siege.

SDF forces reached the northern and eastern gates to Raqqa last week after intense clashes under the cover of U.S.-led airstrikes.

Talal Sillo, a spokesman for the SDF, said Tuesday that the “great battle” is underway. “Morale is high and military readiness to implement the military plan is complete, in coordination with the U.S.-led coalition,” he told reporters in northeast Syria, flanked by representatives of Kurdish male and female fighting units, as well as Syrian rebel groups and Arab tribesmen.

Washington’s decision to back a Kurdish-led force has soured relations with Turkey, a NATO ally, which is battling Kurdish militants within its own borders. In Ankara, Prime Minister Binali Yildirim said Tuesday that the army is not ruling out military force if the battle for Raqqa is seen as a threat to Turkey.

The U.S.-led coalition emphasized the SDF’s “multi-ethnic” composition in its statement Tuesday. But Arab fighters within the SDF have long maintained that they are the junior partner in a force dominated by battle-hardened Kurdish fighters. 

“Sometimes we feel like we are decorations. None of the Arab forces have any real power in this battle,” one man said in a recent interview, speaking on the condition of anonymity because he was not authorized to talk to the media.

The Britain-based Syrian Observatory for Human Rights monitoring group said Tuesday that the SDF has captured some buildings in the Mishlab area and that Islamic State fighters have withdrawn from parts of the district. A second attack was reported against a former Syrian army base on the northern outskirts of Raqqa that is controlled by the Islamic State.

“It’s hard to convince new recruits that ISIS is a winning cause when they just lost their twin ‘capitals’ in both Iraq and Syria,” Townsend said in the coalition statement, using an acronym for the Islamic State.

Islamic State fighters are also under pressure across the border in Iraq, where U.S.-backed forces are locked in a grinding battle to retake the last neighborhoods held by the extremist group in the sprawling city of Mosul.

The group’s defeats across the two countries are believed to have strained its finances, which were heavily dependent on the ability to tax and extort populations under its control.

As Islamic State forces dig deep across their remaining territory, civilians have increasingly been caught in the crossfire, killed by the militants’ bombings and land mines as well as by coalition airstrikes and SDF shelling.

The International Rescue Committee said Tuesday that it was “deeply concerned” for Raqqa’s ­civilians and warned that they risked “facing the full brunt of the assault to come.”

Separately on Tuesday, the Pentagon said it had launched a second airstrike in a month against pro-government forces that it said threatened U.S. and allied troops based in southern Syria.

“Despite previous warnings, pro-regime forces entered the agreed-upon de-confliction zone with a tank, artillery, anti-aircraft weapons, armed technical vehicles and more than 60 soldiers,” the coalition said in a statement. It was not clear how many pro-government troops had been killed.